Conventionally, there are known disposable injector outer tubes produced by injection molding of a synthetic resin such as polypropylene, polycarbonate, and polymethylpentene (TPX). These synthetic resins are inactive for a medicine liquid. The polycarbonate and TPX have an advantage that they can be sterilized by a high-pressure vapor and have a high transparency.
As shown in FIG. 1, the injector outer tube 1 has a barrel portion 2, a lure-tapered portion 3 arranged at a front end of the barrel portion 2, and a flange portion at a rear end of the barrel portion 2 for holding by fingers. The injector outer tube 1 is normally produced by injection molding by using a metal mold 11 consisting of a fixed mold 13, a movable mold 14, and a core pin (core) 15.
The fixed mold 13 has a hole portion 12 having a shape of the outer shape of the injector outer tube 1. The hole portion 12 is provided vertically on a parting line 16 with the movable mold 14. Moreover, the core pin 15 is adapted to pierce the movable mold 14 to be inserted into the hole portion 12. Furthermore, a molten resin is injected into a cavity 17 formed by the hole portion 12, the movable mold 14, and the core pin 15. The cavity 17 consists of a barrel forming portion 18 and a lure-taper forming portion 19 formed by the core pin 15 inserted into the hole 12 and a flange forming portion 20 formed between the hole portion 12 and the movable mold 14.
It should be noted that in the metal mold 11, the fixed mold 13 has a sprue, a runner, and a gate (not depicted) for guiding a molten resin to the cavity 17.
Injection of a molten resin into the cavity 17 is performed as follows, for example. As shown in FIG. 7A, a plurality (three in the figure) of chip gates 31 are arranged at the front end of the barrel forming portion 18; as shown in FIG. 7B, a plurality (two in the figure) of chip gates 31 are arranged at the base of the lure-taper forming portion 19; or as shown in FIG. 7C, a plurality (two in the figure) of pin point gates 31 are arranged at the front end of the barrel forming portion 18. It should be noted in FIG. 7A to FIG. 7C, for explanation, the barrel forming portion 18, the lure-taper forming portion 19, the runner 23, and the gate 31 arranged inside the fixed mold are exposed.
By using the method of FIG. 7A to FIG. 7C, it is possible to obtain a metal molded product without performing cutting the gate 31 after injection of the molten resin.
However, when the methods of FIG. 7A to FIG. 7C are used, the resin is supplied from the front end side of the core pin 14 into the cavity 17 under a high pressure and accordingly, there is a problem that the core pin 15 is inclined, making the thickness of the barrel portion 2 uneven. Moreover, when the resin portions injected from the plurality of gates 31 join in the cavity 17, a distortion may be caused, deteriorating the circularity of the cross section of the barrel portion 2.
Instead of the methods of FIG. 7A to FIG. 7C, there is also a method to provide only one gate 31 at the rear end side of the barrel forming portion 18 as shown in FIG. 8. According to the method of FIG. 7, there is no confluence of the injected resin in the cavity 17, and hence it is possible to prevent the aforementioned distortion.
However, in the method of FIG. 8, when the resin filled in the cavity proceeds along the core pin 15, the flow front F of the resin proceeds earlier at the side where the gate 31 is provided and the flow front F is delayed at the opposite side of the gate 31. For this, as shown in FIG. 9, in the injector outer tube 1 obtained by the method of FIG. 8, the barrel portion 2 has a thicker wall at the side where the gate 31 is provided, which deteriorates the circularity of the cross section of the barrel portion and warp is caused.
Moreover, the injector outer tube 1 is used in combination with a plunger (not depicted) sliding along the inner wall of the barrel portion 2. When the plunger has a gasket made from a thermoplastic elastomer, if the circularity is low, the gasket is creep-deformed, causing a problem that a liquid may leak when the injector is used.